Many high-performance secondary battery applications require accurate estimation of available power supplied by the secondary battery. For example, in hybrid electric vehicles (HEV) and electric vehicles (EV), the vehicle controller continuously requests a battery management system (BMS) for update information on discharge power being supplied from the secondary battery to the motor and charge power that can be supplied to the secondary battery by way of motor or regenerative braking.
The Hybrid Pulse Power Characterization (HPPC) is the power calculation technology widely known in the art. The HPPC can be found in the Partnership for New Generation Vehicles (PNGB) Battery Test Manual (Rev. 3, February 2001) published by the Idaho National Engineering and Environment Laboratory of the U.S. Department of Energy.
The HPPC estimates secondary battery power by solely considering the operation design limits for voltage. Accordingly, this method does not consider the design limits associated with the state of charge (z) and currents of the secondary battery. The “state of charge” as used herein refers to a relative ratio of currently remaining capacity relative to the capacity of the fully-charged secondary battery.
The HPPC models the voltage of the secondary battery simply by Equation 1 below.V=OCV(z)−RI  <Equation 1>
where, OCV(z) denotes a corresponding open circuit voltage of the secondary battery when the state of charge of the secondary battery is z, and R is a constant representing internal resistance of the secondary battery. The parameter z representing the state of charge has a value ranging from 0 to 1.
FIG. 1 schematically illustrates the concept of determining discharge power of the secondary battery using HPPC.
As illustrated in FIG. 1, the voltage Vdis of the secondary battery is measured after the secondary battery in state of charge zk is discharged for a predetermined time (e.g., 10 sec) with a constant current having a value Idis. For convenience of description, Vdis is hereinafter referred to as a “discharge termination voltage”.
Next, the slope of I-V profile, Rdis, is determined based on Equation 1, and by using Rdis as determined, the linear equation, V=−Rdis*I+OCV(zk), is determined regarding the I-V profile. The extrapolation is then applied to the determined equation such that the current value corresponding to Vmin, the lower limit of the discharge voltage, is determined. In this manner, the maximum discharge current Imax,dis is determined.
According to HPPC, when the maximum discharge current Imax,dis is determined, the discharge power Pdis is then determined with Equation 2 below.
                              P          dis                =                                            V              min                        ⁢                          I                              max                ,                dis                                              =                                    V              min                        ⁢                                                            OCV                  ⁡                                      (                                          z                      k                                        )                                                  -                                  V                  min                                                            R                dis                                                                        <                  Equation          ⁢                                          ⁢          2                >            
Meanwhile, the conventional discharge power calculation method described above has several shortcomings.
First, HPPC does not set the operation design limits for current. When the maximum discharge current Imax,dis of the secondary battery determined by HPPC is greater than the operating current upper limit Ilimit,dis of the secondary battery, the discharge power is determined to be greater than the performance of the secondary battery. When this happens, the secondary battery will be discharged under a harsher than actual condition. Accordingly, this can be a threat to safety. In particular, the lithium secondary battery has a risk of explosion.
Further, HPPC assumes that the discharge resistance Rdis corresponding to the slope of the I-V profile is a constant. However, the I-V profile can have non-linearity depending on types of secondary battery. In this case, the discharge power is also set to be greater than actual performance of the secondary battery.
FIG. 2 illustrates an example in which the I-V profile has non-linearity.
Referring to the example of FIG. 2, the discharge termination voltage of the secondary battery is measured to be Vdis when the secondary battery is discharged with the discharge current Idis for a predetermined time. The maximum discharge current Imax,dis calculated by using the discharge termination voltage and the discharge current measured by HPPC as described above becomes greater than the actual value I*max,dis of the maximum discharge current. This is because Imax,dis, calculated with the extrapolation based on an assumption that the I-V profile has linearity, is greater than the value of the current I*max,dis at the actual point of intersection between the I-V profile and the straight line V=Vmin.
Accordingly, there is demand for a new discharge power estimation technology in the related field, which can overcome the shortcomings of HPPC mentioned above.